1. Field of the Invention
This invention relates to a high molecular weight silicone compound (or silicone polymer) having acid labile groups of at least one type and crosslinked with crosslinking groups having C--O--C linkages within a molecule and/or between molecules. It also relates to a chemically amplified positive resist composition comprising the silicone polymer as a base resin which has a high sensitivity to actinic radiation, can form resist patterns by developing with aqueous base, and is thus suitable for fine patterning. It further relates to a patterning method using the composition.
2. Prior Art
As the LSI technology tends toward higher integration and higher speed, further refinement of pattern rules is required. The current patterning technology mostly relies on light exposure which is now approaching the essential limit of resolution which is dictated by the wavelength of a light source. It is generally recognized that in light exposure using g-line (wavelength 436 nm) or i-line (wavelength 365 nm) as a light source, a pattern rule of about 0.5 .mu.m is the limit. For LSIs fabricated by such light exposure technique, a degree of integration equivalent to 16 mega-bit DRAM is the limit. At present, LSIs fabricated in the laboratory have reached this stage. It is urgently required to develop a finer patterning technique.
Since Ito et al. proposed a chemically amplified positive resist composition comprising a polyhydroxystyrene resin whose hydroxyl group is protected with a tert-butoxy-carbonyloxy group (t-Boc group), known as PBOCST, and a photoacid generator in the form of an onium salt, a number of high sensitivity, high resolution resist compositions have been developed. Although these chemically amplified positive resist compositions have high sensitivity and high resolution, formation of a fine pattern having a high aspect ratio is deemed difficult when the mechanical strength of the resultant pattern is taken into account.
A number of chemically amplified positive resist compositions using polyhydroxystyrene as a base resin and having sensitivity to deep-UV, electron beams and X-rays are known in the art. These resist compositions relay on a single layer resist technique although a two-layer resist technique is advantageous in forming high aspect ratio patterns on stepped substrates. Because of such outstanding problems of substrate steps, light reflection from substrates, and difficult formation of high aspect ratio patterns, the known resist compositions are far from practical.
It is known that the two-layer resist technique is advantageous in forming high aspect ratio patterns on stepped substrates. To enable alkali development in the two-layer resist technique, silicone polymers having hydrophilic groups such as hydroxyl and carboxyl groups are required. Since the silicones having a hydroxyl group directly attached thereto, however, undergo crosslinking reaction in the presence of acid, it is difficult to apply such silanols to chemically amplified positive resist materials.
While polyhydroxybenzylsilsesquioxane is known as a stable alkali soluble silicone polymer, its derivatives obtained by protecting some hydroxyl group with t-Boc groups form chemically amplified silicone system positive resist materials when combined with photoacid generators as disclosed in Japanese Patent Application Kokai (JP-A) No. 118651/1994 and SPIE, Vol. 1952 (1993), 377.
However, when a resist composition comprising as a base a silicone polymer in the form of such a polyhydroxybenzylsilsesquioxane whose hydroxyl groups are partially protected with t-Boc groups is used to form an upper layer resist film, there arise the problems that a footing phenomenon occurs at the interface with the lower layer film, and the upper silicone resist film tends to form a difficultly soluble subsurface stratum. The footing phenomenon and the difficultly soluble subsurface stratum restrain the precise dimensional control of the pattern of the resist film, which is inadequate for fine processing.